I suggest 24 Oct. 1929 ('Black Thursday'), the Wall St. crash, which plunged the world into deep recession.
Also 28 June 1914, the assassination of Archduke Franz Ferdinand in Sarajevo, precipitating the First World War.
1348 was a grim date too: the worldwide pandemic of the Black Death (pneumonic plague).
August 1945 saw the first use of nuclear weapons.
The prehistory invention of the wheel was pivotal (!).
Also 1848, the first use of anaesthetics.
Britain's Industrial Revolution was a collective superidea
which changed the world dramatically.
More suggestions if you wat them......